Computed Radiography (CR) involves readout-scanning of a photostimulable storage phosphor plate (CR-plate) previously exposed to an X-ray source in an imaging mode. The technology was developed as an alternative to film-based radiography and has been used for decades in the medical and veterinary areas. The medical readout devices use flying-spot laser scanners with large and complex optical assemblies in bulky chassis designed for an office environment. These devices are not suitable for the man-portable, field applications intended for computed radiography products such as the Future Radiography System (FRS) and the Real Time Radiography (RTR) system products developed by Applicant, Leidos, Inc.
The Leidos FRS products are the only known CR systems compatible with field operations that also meet the form-factor requirements of the military and first-responder customers. The limiting form factors are the overall height (thickness) of the cassette/reader system, about 1.5 inch or less, plus the requirement for minimum dead space on two sides at a corner.
An existing Leidos product is based on a scan head described in U.S. Pat. No. 7,825,394 (hereafter the '394 Patent”), the substance of which is incorporated herein by reference in its entirety. The prior art scan head described in the referenced patent uses a linear array of charge-coupled device (CCD) photosensors placed very close to the CR plate to enable contact-transfer (sometimes referred to as proximity focusing) of the violet, stimulated light to the readout pixels. There are no imaging optics between the CR plate surface and the CCD array—only a thin filter and a protection film. The filter is required to eliminate the red stimulation light from the violet CR plate emission and is approximately 250 microns thick (est.). The protection film, typically 60-70 micron thick Mylar, protects the scan head, reduces damage to the fragile CR plate and provides a smooth-running interface. The total height of the head assembly is about 10 mm which is quite compact and helps meet the overall thickness goal. The width of the head is also about 10 mm although this dimension is less critical as long as the design supports the minimum dead-space requirement at one edge. The active length of the head is 14 inches with only a few mm of dead space on each end for coupling to the mechanical scan drive.
Red-light stimulation of the CR plate is accomplished using a dense array of LEDs aimed at the plate along the length of the prior art head. The LEDs are separated from the CCD array by a thin metal wall extending all the way down to the CR plate surface. This “knife-edge” interface results in thin line-source of red light that is created by scatter-diffusion through the 290 micron thick CR plate around to the region viewed by the CCD array. An advantage of this simple geometry is that the CCD array can be positioned directly against the knife-edge wall ensuring good alignment relative to the line-source emission. An additional advantage is that the flood-illuminated region directly under the LEDs affords effective erasure of the CR plate during the primary scan and also during the return-to-home of the head, if the LEDs remain energized. Obviously, the head can run the primary scan in only one direction.
The system runs in a pushbroom-style scan imaging mode taking about 25 seconds to travel the 17-inch length of the CR plate. The image is divided into square pixels 172 microns on a side. There are 2400 pixels in the 17-inch direction (direction of mechanical translation in FRS). Hence, the read-integration time for each pixel is about 10.5 ms. The LED power must be adjusted to bleach out more than 90% of the stored violet photons in this time interval, but not too high such that additional bleaching occurs outside the photosensitive collection region in the scan direction. Other than this requirement, the only limit on scan speed is the readout time of the CCD array and the capabilities of the mechanical drive system.
By way of example only, the CR plates are very high quality and may be supplied by a manufacturer such as Carestream Health, Inc. The CR plate is a highly diffusive granular phosphor material that is flat, smooth, and appears bright-white in room lights. The current FRS system uses the Carestream General Purpose or GP plate providing the highest X-ray stopping power.